Most conventional prescription lenses have relatively flat base curves. Such lenses provide a limited field of view due to peripheral distortion and/or physical size limitations. Their relatively flat shapes limit the amount of eye protection afforded by the lenses, particularly near the temples.
Wrap-around eyewear has been developed in an attempt to provide wider fields of view and greater eye protection. Wrap-around design also permits different and sometimes striking overall styles for the eyewear. However, wrap-around eyewear is typically non-prescription. These products also typically have flat base curves between 6 and 10 D. Wrap (and sometimes rake) are achieved by rotating and/or translating the optical axes of the lens in the as worn orientation. See e.g. U.S. Pat. No. 1,741,536 to Rayton; U.S. Pat. No. 5,689,323 to Houston et al. This causes the line of sight of the wearer to deviate from the optical axis, and optical performance is often significantly degraded. Peripheral vision is typically poor.
Early in the history of ophthalmic science, steeply curved prescription lenses had been described, although not as vehicles for providing greater field of view or eye protection. A relationship between curvature and through power is shown in the so-called "Tscherning's" ellipse. First described nearly 100 years ago, it attempts to identify combinations of lens curvature and lens power which have minimum abberation. The general form of the Tscherning ellipse is shown in FIG. 1. FIG. 1 is given for assumed typical values for lens parameters such as index of refraction, vertex distance, lens thickness, etc. The Tscherning ellipse retains its ellipsoid shape and inclined orientation for various assumed values of lens parameters, while the precise location of points on the ellipse may change. The ellipse of FIG. 1 is derived from the corrected von Rohr equation (after Morgan) solved for point-focal (zero astigmatism) distance vision.
The lower portion 10 of the ellipse is the so-called "Ostwalt section" which describes a selection of relatively flat front surfaces for lens powers typically used in conventional prescription ophthalmic lenses. The upper portion 12 of the curve, called the "Wollaston section", describes much more steeply curved lenses which have never gained acceptance as lens forms, although there are historical instances of attempts to make such objects (e.g. Wollaston himself). See, M. Jalie, The Principles of Ophthalmic Lenses p. 464 (4t Ed. London, 1994). Because of difficulties in fabrication, such early lenses were probably of small aperture and, consequently, perhaps, regarded as unacceptable for cosmetic reasons and because of their limited field of view.
Modern lenses with steeply curved front spherical surfaces have been made for the treatment of aphakia (absence of the natural lens of the eye as in the case of surgical removal of the lens). The general form of these lenses is shown in FIG. 2. See M. Jalie at p. 151. Such lenses serve essentially as an eye lens replacement and are characterized by great thickness and high plus power (greater than +5 D and typically +12 D or greater). The aperture A of these lenses are of small size e.g. 26 or 28 mm in diameter. Typically such aphakic lenses have a plano radial flange 14.
Today, the vast majority of conventional prescription lenses are relatively flat, single vision, Ostwalt section, miniscus lenses which are glazed like window panes into flat outline spectacle frames.